1. Field of the Invention
The present invention relates to a semiconductor integrated circuit including a MOS transistor for generating a reference voltage.
2. Description of Background
FIG. 1 shows a conventional semiconductor integrated circuit for generating a reference voltage. This circuit includes a resistor 1 connected at one end to a power source Vcc, a first MOS transistor 2 formed with channel ion-implantation and serving as a first diode having an anode connected to the other end of the resistor 1, and a second MOS transistor 3 formed with channel ion-implantation and serving as a second diode 3 having an anode connected to the cathode of the first MOS transistor 2 and a cathode connected to a power source at ground potential. The node between the resistor 1 and the first MOS transistor 2 is connected to an output terminal 4.
In this semiconductor integrated circuit, the sum of a forward voltage drop VF of the first MOS transistor 2 and a forward voltage drop VF' of the second MOS transistor is outputted from the output terminal 4. Thus, this circuit has been conventionally used when it is desired that a voltage greater than the forward voltage drop of one diode be outputted from the output terminal 4. Namely, a plurality of diodes are connected in series, as shown in FIG. 1, and the sum of the forward voltage drops of the diodes is outputted from the output terminal 4.
In this semiconductor integrated circuit, however, the voltage at the output terminal 4 varies easily, depending on the operating temperature. Further, the characteristics of this type of semiconductor integrated circuits vary depending upon manufacturing conditions, and it is difficult to attain the desired output voltage with high precision. Very small variations in the manufacturing conditions lead to non-uniformity in the forward voltage drop of the manufactured products, and the forward voltage drop of the diode varies considerably as a function of the operating temperature. Furthermore, since a number of such diodes are employed in order to produce an output voltage greater than the forward voltage drop of one diode, variations in characteristics of manufactured semiconductor integrated circuits increase, and the output voltage of the circuits vary accordingly.